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What's an AUMF? Obama says it's why he can drop bombs in Syria.

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(CNN)American warplanes are hard at work over Iraq and Syria, helping Iraqi army and Syrian rebel forces battle ISIS. In Iraq, American troops are busy training the military.

All of this is being done -- and will continue to be done -- without being officially sanctioned by Congress, although President Barack Obama now says he needs an Authorization for the Use of Military Force -- AUMF.

Passing AUMF legislation is Congress' way of giving the President permission to engage in prolonged military operations. Under the War Powers Resolution of 1973, the President can send U.S. forces into a conflict -- in this case airstrikes -- but must end those hostilities after 60 days unless Congress has given specific authorization.

The U.S. launched airstrikes in Iraq in August at the invitation of the Iraqi government, in part to protect American interests. That air campaign did not extend to Syria until September. In speaking to the American people about the fight against ISIS early that month, Obama said he had the authority he needed to act, but would welcome Congress' approval.

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"I have the authority to address the threat from ISIL, but I believe we are strongest as a nation when the President and Congress work together," he said on Sept. 10. "So I welcome congressional support for this effort in order to show the world that Americans are united in confronting this danger."

The White House argued the 2001 AUMF, which authorized military force against the groups who were either responsible for the September 11th attacks or who harbored the groups responsible -- i.e. al Qaeda, the Taliban and associated forces -- applies in this case. Why? Because ISIS was once a part of al Qaeda and considers itself to be the true inheritor of Osama bin Laden's legacy.

Not everyone agreed with that rationale, including members of the President's own party, like Virginia Sen. Tim Kaine.

"Using a 13-year-old authorization crafted in different times for a different circumstance under a different administration for a different bit of geography with the support of a vastly different Congress to justify a new war 13 years later is the not the way that the nation should make the grave decision about whether to go to war," Kaine said on the Senate floor on Dec. 16.

Fast forward to the State of the Union address on Jan. 20.

"Tonight, I call on this Congress to show the world that we are united in this mission by passing a resolution to authorize the use of force against ISIL," the President said. "We need that authority."

Administration officials say they are consulting with members of Congress and planning to send draft language for the authorization to Capitol Hill, but there's still no timeline for when that language will arrive at the other end of Pennsylvania Avenue.